Airborne particles exist in a wide variety of shapes and sizes. Aerosols are composed of either solid or liquid particles. Conversely, gases are molecules that are neither liquid nor solid and expand indefinitely to fill the surrounding space. Both types of contaminates exist at the micron and sub-micron level in air. Most dust particles, for example, are between 5-10 microns in size (a micron is approximately 1/25,400th of an inch). Other airborne contaminates can be much smaller. Bacteria commonly range anywhere between 0.3 to 2 microns in size, and viruses can be as small as 0.02 microns in size or smaller. The importance of removing these contaminates varies based upon the application. Semiconductor clean rooms and hospital operating rooms are two examples of spaces where the ability to remove contaminates is critical.
One factor complicating the removal of contaminates is that particle number density increases with smaller particle size. For example, in the typical cubic foot of outside air there are approximately 1000 10-30 micron sized particles. The same volume of air, however, contains well over one million 0.5 to 1.0 micron particles. As particle measuring instrumentation evolve they are capable of measuring deeper into the submicron range. Thus, advances in particle detection technology has confirmed that a great majority of all airborne particles are less than a micron in size. The prevalence of small particles is problematic from an air quality standpoint because small particles are hard to control and therefore hard to capture. Yet most contamination problems are caused by small particles.
Most small particles have a charge associated with them, while larger particles tend to be more neutral in charge. Thus, the movement of small airborne particles is primarily governed by electromagnetic forces, whereas the movement of large airborne particles is primarily governed by airflow. Further, small particles are also more influenced by Brownian Motion, both thermal and kinetic. However, larger particles have more mass associated with them. This is the basis of why larger particles are controlled more by the airflow generated by an HVAC system.
Particles acquire charge by three basic mechanisms. Diffusion charging occurs when particles are charged by random collisions between ions and other particles. The motion and collisions result from a process known as Brownian motion. The particle can take on multiple charges by this mechanism. Field charging occurs when rapid ion movement in an electric field causes frequent collisions between ions and particles. Finally, static electrification occurs when particles are separated from surfaces, thereby charging the particles. The factors that affect how a particle behaves in an electric field include particle size, the charge associated with the particle, and the strength of the electromagnetic field. The smaller the particle, the more it is influenced by an electromagnetic field. The more charge there is on a particle, the stronger the influence of the electromagnetic field. The stronger the electric field, of course, the more influence it has on the particle.
As discussed above, a great majority of the airborne particles found in nature are less than a micron in size. Thus, conventional air filtration systems that utilize airflow to capture airborne particles by trapping them in a filter device inevitably fail to trap smaller particles, leaving them free to circulate within an occupied space. Furthermore, the more efficient the filter in a system governed by airflow, the greater the pressure drop across the system. This pressure drop consequently decreases the efficiency of air filtration systems dependent on airflow as the primary force on airborne particles.
To overcome the difficulties associated with the capture of small particles, different particle conditioning techniques can be orchestrated together to control the transport, capture, and deactivation of particles. These conditioning tools include but are not limited to, particle ionization, particle polarization, and controlled particle colliding.
Particle Ionization—
Particle ionization occurs when a particle passes through an ion field. One type of ion field is a corona field. A corona field is created when a voltage is passed through a very thin wire or a thin metal blade with a serrated edge. Upon application of the voltage, electric fields concentrate on a sharp point and on a thin edge. When the electric field is strong enough, charges are emitted to the surrounding space, thereby developing a space charge. For example, if a negative high voltage is applied to a thin wire or metal edge, electrons are emitted to the air surrounding the wire or blade. When a particle passes through this created electron field, the particle picks up, or acquires, some of the electrons and becomes a negative ion (this also applies to a positive field which produces a positive ion). In the case of a particle passing through the negative ion field (electrons) the particle becomes negatively charged, thereby allowing it's movement to be controlled by the subsequent application of another electric field. If a grid that has the same voltage applied to it as the corona grid is placed in the path of the particle, the particle will be repelled by the grid (like charges repel each other). Furthermore, if a positive wire is placed downstream from the negative wire the conditioned particle will be propelled towards this positive grid (unlike charges attract each other). This is how the trajectory of particles can be controlled using precisely controlled electromagnetic, electrostatic, and/or electrodynamic fields.
Particle Polarization—
When a particle passes through a strong electrostatic field it can form a dipole, wherein one end of the particle is positively charged and the other end is negatively charged. This polarization is due to the fact that opposite charges attract and like charges repel. When a particle approaches a strong electrostatic field, such as a −15 kV field, a dipole is formed. Some of the positive charges in the particle will move toward the strong field (front of the particle) and some of the negative charges will move towards the opposite end (rear) of the particle, away from the static field (FIG. 2A). Once this occurs the particle passes through the electrostatic field. If a second electrostatic field, of the opposite potential is downstream from the first electrostatic field the particle propels toward it.
Controlled Particle Colliding—
Controlled Particle Colliding performs at least two functions. First, it causes collisions between sub-micron sized particles to form larger particles, thus changing them from being dominantly controlled by electromagnetic fields to being controlled by airflow. Second, it makes particles neutral in charge. Particles will not only stay entrained in the airflow without being influenced by the electromagnetic fields in the room environment, but will not be as likely to form strong bonds with surfaces and objects in the room, even if they should come in contact with them.
Media Filter Systems—
This major class of filter system typically uses no electromagnetics in its operation. Basically this type of air cleaning device is a particle block. The particles that get to the device are filtered in the media material. In other words filtration occurs at the filter. These devices are placed in the airstream perpendicular to airflow. Airflow brings the incoming particles to the filter and the incoming particles get trapped as the air passes through the filter. This type of device depends on airflow.
Collector Systems—
When proper dielectric media material is utilized and an electrostatic field is applied across the media material, an opposite polarizing electric field is generated across the media material causing the material itself to polarize (see FIGS. 4 and 5). Depending on the density of the media material determines the depth of penetration of conditioned particles. Optimizing the Collector and proper conditioning of particles results in efficient particle collection (and deactivation of microbes where appropriate).
Transport Mechanisms are what control the movement of particles. In every building environment there are forces present that determine these transport mechanisms. The Dominant Transport Mechanisms in a building environment are Airflow and/or Electromagnetic Fields, as described herein. Only relatively large particles, greater than a micron in size, are controlled by airflow. Smaller particles are dominantly controlled by electromagnetic fields. The smaller the particle, the more this statement becomes true.
Two equations dictate particle behavior. 1. Force equals the change in momentum of the particle (F=ma), due to airflow. 2. Charge times the electric field E (F=qE) due to electric forces in the room environment. Note 1: F is the force, m is the mass, a is acceleration, and E is the electric field. The first equation (F=ma) describes how airflow controls particle behavior and the second equation (F=qE) describes how the electric field controls particle behavior.
As is known in the art, the difficulty associated with capturing small airborne particles can be overcome by utilizing Particle Accelerated Collision Technology™ (PACT) (U.S. Pat. No. 7,175,695). PACT makes airflow the dominant transport mechanism and controls the behavior of fine particulates by creating inelastic collisions on a sub-micron level. This causes smaller particles to collide inelastically, thus becoming larger, thereby enabling any associated filtration system to easily remove these larger particles from the air. This collision process significantly improves the ability of a standard filtration system to remove and reduce indoor and outdoor generated contaminate levels.
Controlled Particle Colliding is similar to PACT, but much more compact. By combining it with the other components described herein it is made as effective as PACT. Alone, it would not be as effective due to its depth of influence being much smaller than an actual PACT system.
Also known in the art is Particle Guide Technology (PGT) (Pub. No. 2012/0085234). PGT forces particles to travel in a desired manner to a desired location, and/or a Particle Collector. The Particle Collector then traps the particles, removing them from the occupied space. PACT and PGT both utilize controlled electromagnetic fields to guide particles to a desired location. They are employed as a particle control device.
The majority of present filtration devices depend on airflow to guide particles to the filtration system. In general they are particle traps. Further, the space available in a typical HVAC system is limited. When the space that the filter is placed in is limited (in the direction of depth) the efficiency and/or pressure drop of the system can be compromised. Although great strides have been made in the efficiency of the traps, little has been done to control the particle itself. It should be mentioned that different particle conditioning techniques have been utilized individually to enhance particle filtration. However, to combine these effects in an optimized manner to control particle behavior is the goal of the present invention. By conditioning and controlling particles, the present invention takes advantage of the dominant transport mechanisms in air.